In digital integrated circuit (IC) technology, there is a drive for reduction of size. However, as the size of the IC shrinks, signal lines may be placed very close to each. As the space between lines decreases, they begin to interact due to parasitic capacitances. Consequently, the lines do not behave as independent signal carriers.
For example, a signal line situated between two other lines has several associated parasitic capacitance. It first has it own self capacitance between the substrate and itself. In addition, there is a coupling capacitance between the center line and each of the adjacent lines. If the center line is floating, i.e. the charge on the line is stored on the line's capacitance, then changes on either or both of the adjacent lines will cause noise to be injected into the middle line. The amount of noise is linearly dependent on the value of coupling capacitance, which in turn is inversely dependent on the space between the lines. Therefore as digital ICs become more dense and lines are placed closer to each other noise injection problems intensify.
One type of bus in digital circuits that is particularly sensitive to capacitive coupling noise is frequently referred to as a tri-state bus. In general, a tri-state bus is a bus that is driven by one or more tri-state drivers. Only one of the drivers is selectively enabled to drive the bus. Once the tri-state bus is charged to a certain logic level, the selected driver is disabled. Thus, the tri-state bus is floating and may be sampled by other logic circuits within the digital circuit. Since the bus is floating, (i.e. not continuously driven by any other circuitry), it is more susceptible to noise. For this reason tri-state buses are connected to leakers.
Leakers provide noise suppression for the tri-state bus. A typical leaker is comprised of two inverters having their inputs connected to the others output. In addition, an input of one of the inverters is connected to the tri-state bus. The magnitude of high or low voltage fluctuations due to noise that the leaker can tolerate is determined by its "trip" point. The "trip" point of the leaker is approximately equal to half of the voltage swing between the high and low voltage level of its associated logic signal. Consequently, the leaker can tolerate voltage fluctuations that are less than half of it associated voltage swing.
For example, if a negative voltage fluctuation (having a magnitude of less than the leaker's trip point) occurs on a tri-state bus which is charged to a high voltage level, the leaker functions to drive the tri-state bus back to the high voltage level. However, if the voltage fluctuation is of a large enough magnitude, it may cause the leaker to change states, i.e. drive the tri-state bus from one logic level to another. And, if the tri-state bus is sampled by another circuit, erroneous data would be transferred.
Thus, the noise suppression capability of the leaker is limited to a noise margin. Further, when buses are placed very close together, as is typical in current digital integrated circuit designs, the magnitude of noise generated by the capacitive coupling effects increases. As a result, the leaker becomes even more ineffective.
The circuit of the present invention provides superior noise suppression in comparison to the leaker as described above. It is capable of differentiating between voltage transitions resulting from noise and intentional voltage transitions generated by the tri-state drivers. In addition, the noise suppression circuit of the present invention is not limited by the magnitude of the noise.